Reduction in cooldown time for cryogenic refrigerator

ABSTRACT

Cryogenic refrigerator with a free piston displacer has an adjustable stop to control its stroke. Longer strokes during higher temperature operation produce faster cooldown rate and, as the design temperature is approached, the stop is adjusted to limit stroke to a value consistent with optimum refrigeration at design temperature.

United States Patent Berry MI Oct. 21, 1975 [5 1 REDUCTION IN COOLDOWN TIME FOR 3.552.120 1/1971 Beale 62/6 CRYOGENIC REFRIGERATOR 3,673309 7/1972 Bamherg 2. (12/6 3,788,772 i/l974 Noble H 62/6 [75] Inv n or: Ro rt L- Berry, Palos Verdes 3.802211 4 1974 Bamberg .1 62/6 Peninsula, Calif.

[73] Assignee: Hughes Aircraft Company, Culver Primary Examiner-"William y City, Calif. Attorney, Agent, or FirmAllen A Dicke Jr; W H. M All I {22 F1led: Mar. 4, 1974 ac H [2!] App]. No.: 448,116 57 ABSTRACT Cryogenic refrigerator with a free piston displaccr has 2% 4 1 62/9467 an adjustable stop to control its stroke. Longer strokes 5b during higher temperature operation produce faster 1 le 0 earc t 1 1 i i. Cooldown rate as the design temperature i p preached, the stop is adjusted to limit stroke to a [56] References C'ted value consistent with optimum refrigeration at design [empera[ure 2,906,101 9/1959 McMahoy 62/6 3,296,808 H1967 Malik 62/6 7 Clamst 4 D'awmg 44 L 1 1 2 IO 42 m 2O 3 N 1 l2 l i U.S. Patent 0a. 21, 1975 Fig. 2.

Fig. 3.

Fig. 4.

REDUCTION IN COOLDOWN TIME FOR CRYOGENIC REFRIGERATOR BACKGROUND OF THE INVENTION This invention is directed to a cryogenic refrigerator which has a length of stroke control on its expander piston, and particularly those refrigerators which have an expander piston which is not controlled by a crank.

Cryogenic refrigerators are usually designed to produce a particular cryogenic temperature at a cold spot and maintain that temperature for a particular cryogenic heat load. Several previous methods are known to increase the cooldown rate. Increase of refrigerator speed does provide an increase in the cooldown rate, because the refrigeration cooling rate is generally proportional to the number of refrigerator cycles per unit time. Hence, by doubling the speed of the refrigerator, the cooling rate will initially be doubled, as long as the losses are not too severe, which is usually the case near room temperature; however, doubling the cyclic rate, even at room temperature, means that flow rates are doubled and thus there are greater pressure drop losses. This means that larger and heavier equipment is required. In some applications, it is only necessary to increase the cyclic rate of the expander. This is done in cases where mechanical valving isolates the speed of the expander from the speed of the compressor and where the capacity of the compressor under normal conditions is sufficient to supply the expander, even while running at an increased speed; however, such an increase in expander speed results in sufficiently greater fluid power comsumption by the expander. For any system where the expander is speeded up, it is necessary to use either double windings on the motor, some type of variable mechanical guide, or some type of electrical power control to change the speed and control the speed of the compressor and expander during the cooldown time. Normally, these structures result in increased size of the components.

The method and device for reduction of cooldown time in accordance with this invention does not necessitate the increase of size of any of the components. The structure of the expander section in accordance with this invention does not result in a significant size or weight increase.

SUMMARY OF THE INVENTION In order to aid in the understanding of this invention, it can be stated in essentially summary form that is directed to a method and device for reduction of cooldown time for cryogenic refrigerators. In particular, it is directed to a device useful with expander pistons which are not controlled by a crank, but reciprocate to provide refrigeration. It comprises the employment of an adjustable stop which permits longer expander stroke at higher temperatures where greater cooling is achieved and a reduced expander stroke toward cryogenic temperatures where the reduced stroke is more efficient.

It is thus an object of this invention to provide a method and device for reducing the cooldown time of cryogenic refrigerators which have an expander piston which reciprocates and of which the stroke can be conveniently controlled. It is a further object to provide a device for reduction of cooldown time which provides for minimum cooldown time due to increased stroke at higher temperatures so that increased power from the supply source is not required and increased complexity of the supply is not required. It is a further object to provide for increased cooldown rate of a cryogenic refrigerator where no increase in speed or changes in cyclic control are necessary. It is a further object to provide a reduction in cooldown time for modified Stirling and modified Vuilleumier (VM) cryogenic refrigerators.

Other objects and advantages of this invention will become apparent from a study of the following portion of the specification, the claims, and the attached drawing.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1, is a semi-schematic drawing, with parts broken away, and showing a longitudinal section through the expander cylinder of a modified Stirling cycle refrigerator.

FIG. 2, is a longitudinal section through the preferred embodiment of an adjustable stop which controls the stroke of the expander piston.

FIG. 3, is a PV diagram in the cold volume of the expander cylinder.

FIG. 4, is a PV diagram of the conditions above the compressor piston.

DESCRIPTION OF THE PREFERRED EMBODIMENT This invention is directed to method and apparatus for decreasing the cooldown time of particular types of cryogenic refrigerators. It is applicable to those reciprocating machines where the motions of the compressor, whether it be a mechanical or thermal compressor, and the expander are not mechanically linked. Modified Stirling refrigerators are shown in the invention by W. H. Higa, U.S. Pat. No. 3,421,331 particularly FIG. 5, and in G. Prast U.S. Pat. No. 3,487,635. Examples of the modified VM refrigerator are shown in B. S. Leo U.S. Pat. application Ser. No. 447,615 filed Mar. 4, 1974 entitled "Magnetic And Spring Driven Cryogenic Vuilleumier Refrigerator and in B. S. Leo U.S. Pat. application Ser. No. 448,182 filed Mar. 7, 1974 for Vuilleumier cryogenic refrigerator hot cylinder burner head.

In general, modified Stirling and modified Vuilleumier refrigerators have compressor and expander sections which are separated, except for a gas passage between the two sections. Motion of the expander is controlled by pneumatic pulses generated at the compressor or by other nonpositive means. Gross cooling capacity of the expander is directly proportional to the volume swept at the cold end of the expander piston. Gross cooling can be increased by increasing the expander stroke and thereby increasing expanded volume. When the expander is at ambient temperature, there are only minor refrigerator losses in the expander and the cooldown rate is proportional to the gross cooling; hence, increasing the gross cooling by increasing the stroked volume results in an increased cooldown rate. As the machine gets cold, thermodynamic and thermal losses will increase and, at a certain tempera ture the increased stroked volume will become a detriment rather than an advantage. At that time, maintaining the maximum cooldown rate is accomplished by reducing the stroked volume of the expander to its normal value. One of the losses which increases with cooler temperatures is the shuttle loss of the expander piston. The exact temperature at which the transition from increased stroke to normal stroke should take place depends on particular design conditions; however. as a rule of thumb reduction from maximum expander stroke to normal expander stroke would be in the order of 2.5 times the desired load temperature. Thus. in cooling down a refrigerator from 300 to 30K, an appropriate stroke reduction to normal would take place at about 75K. On the other hand. if the design temperature is 77K. the transition from enhanced stroke to normal stroke could take palce at about 190 K. Thus. this invention is directed to temporarily increasing the stroke of the expander. Various structures are capable of accomplishing this result. The structure shown in FIGS. I and 2. are a preferred embodiment.

FIG. 1 illustrates a modified Stirling refrigerator l0. Refrigerator I comprises a compressor 12 which has a piston 14 reciprocating in compressor cylinder 16. Reciprocation is caused by crank 18 which is driven by an appropriate motor. This produces pressure pulses of refrigerant gas in line 20.

The expander is generally indicated at 22. The expander comprises an expander cylinder 24 in which is mounted a reciprocating expander piston 26. Volume 28 is the expansion volume in which the refrigerant gas is expanded to produce refrigeration. Cold cylinder head 30 is the point of refrigeration and is the location upon which refrigerated devices are mounted. Window 33 in insulator housing 34 permits the refrigerated device. such as an infrared sensitive device, to have a field of view external to the insulaator housing. The insula tor housing can be in the nature of a dewar or the like. Volume 36 is an ambient temperature volume to which the line is connected. Volume 36 is connected through the interior of cold piston 26 through regenerator 38 to cold volume 28. Sliders or guides on the exterior of the expander piston permit the piston to slide within the expander cylinder. There is no pressure drop along the length of the expander piston, except for the pressure drop through the regenerator, so that the sliders do not have a substantial seal duty.

Balance piston 40 is mounted on the expander piston and is sealed in balance cylinder 42. The upper end of balance cylinder 42 is spring volume 44.

The pressure in spring volume 44 corresponds to the mean cyclic pressure in volume 28. Thus, cyclic pressure in line 20 causes back and forth motion of the cold displacer 26 within its cylinder, dependent on the pressure in the expansion volume. Cooling is generated in volume 28 by the gas expansion process. High pressure gas is initially admitted to the expander. The gas is then expanded and this depressurization causes the temperature of the gas to drop providing refrigeration.

A simple expander is shown. A two stage expander with a stepped expander piston is the force equivalent because the piston area against which the gas expands is the same even if it is divided into several areas ex panding gas at different temperatures. Thus, a multiple stage expander works the same way.

Referring to the operation of the cycle, FIG. 4 is a PV diagram of the compressor. With a compressor piston at top dead center point 46, the cold displacer is assumed to be at top dead center, as at point 48 which is the maximum cold volume. Since the pressure in the cold cylinder is now higher than the mean pressure in the penumatic spring volume 44, a force is acting on the cold displacer tending to hold it in the top dead center position. Thus, the cold displacer 26 is in a stable position at point 48.

During the first quarter rotation of the crank as com prcssor piston 14 moves toward bottom dead center. the system pressure decreases steadily and reaches the mean pressure at point 50 approximately at the end of this quarter revolution. The corresponding point in FIG. 3 is at point 52 where the cold displacer remains at the top dead center position; however, the pressure which maintains the cold displacer in this position is de creasing. During the next quarter revolution. the compressor piston continues to move toward bottom dead center, and the system pressure continues to drop. When the pressure in the expansion volume decreases below the mean pressure in the penumatic spring volume 44, an activation force is developed, When this force exceeds the frictional drag of the seals. and differ ential pressure forces acting on the displacer. the cold displacer will move towards its bottom dead center position 54, as seen in FIG. 3. Immediately below the point 52. the cold displacer starts to move. and, as the compressor piston I4, decreases to its condition of bottom dead center point 56 the cold displacer 26 moves to its bottom dead center point 54, as illustrated in FIG. 3. At the end of the half cycle. the compressor piston is at bottom dead center and the cold displacer is at bottom dead center. In this position. pressure in the system is near minimum. The cold displacer is again in a stable position. as being held in its bottom dead center position.

During the third quarter rotation of the crank, the compressor piston again moves toward top dead center from point 56 past point 58. During this motion, the system pressure increases steadily and reaches approximately the mean pressure at point 58 of the quarter cycle. The corresponding pressure is shown at point 60 in FIG. 3. During the final quarter cycle. the compressor piston continues to move toward top dead center and the system pressure continues to rise. When the pres sure rises above the mean cyclic value; that is, above points 58 and 60, a resultant force is developed on cold dispiacer 26. When this force exceeds a frictional drag on the seals. and differential pressure forces acting on the displacer, the cold displacer will move toward top dead center, as illustrated at the curve above point 60 in FIG. 3. At the end of this cycle, the cold displacer is at bottom dead center and the compressor piston is at top dead center; thus, the cycle is complete.

The area enclosed by the indicated PV diagram of the expander, FIG. 3. is the work performed by the gas on the cold displacer and is equal to the gross refrigeration developed at the expander. Thermal losses resulting from heat conduction along the cylinder walls and displacer body and shuttling of the displacer, as well as pumping losses, pressure drop losses, and regenerator losses reduce the useful refrigeration to a net value which is less than the gross refrigeration. Some of these losses are functions of the stroke of the cold displacer.

Stroke adjister 62, seen in FIGS. 1 and 2, is located on the expander cylinder to limit the stroke of the ex pander piston therein. As seen in FIG. 2, stroke adjuster 62 comprises solenoid coil 64 in which is mounted movable armature 66. Stop nose 68 is of such dimension to extend out of the soleniod housing. Spring 70 causes stop nose 68 to extend, when solenoid coil 64 is not energized. The spring 70 is of such strength and stop nose is of such dimensions as to properly limit the stroke of the expander piston when the refrigerators are operating under normal refrigeration load conditions; however. when solenoid 64 is energized, stop nose 68 is retracted and the expander piston 26 can operate over a longer stroke to provide a greater expansion ratio in expansion volume 28. With start of cooldown. refrigeration capacity is enhanced by this greater expansion ratio, and thus, at the begining of cooldown, soleniod 64 is energized so that stop nose 68 does not interfere with stroking of expander piston 26; however, as the design temperature is approached, solenoid 64 is de-energized to permit extension of stop nose 68 to reduce the stroke. The appropriate conditions at which the stroke is reduced depends upon design conditions, but is usually found at about 2.5 times the design temperature. Thus, enhanced efficiency of refrigeration is achieved during the cooldown portion to enhance cooldown rate; however, refrigeration efficiency is maintained at design temperatures so that over-design for cooldown capacity is not necessary Other structures than the soleniod construction can be employed to adjust the stroke. For example, a stroke adjuster having more than two positions can provide more than two expansion ratios so that other cooldown circumstances can be achieved. Furthermore, an infinitely variable adjustment is feasible so that the stroke can be continually adjusted to that which causes maximum cool-down rate as the temperature changes and as the losses change in character due to temperature changes. The disclosures referenced above are incorporated herein in their entirety.

This invention having been described in itss preferred embodiment, is clearly suseptible to numerous modifications and embodiments within the ability of those skilled in the art and without the exercise of the inventor faculty. Accordingly, the scope of this invention is defined by the scope of the following claims.

What is claimed is:

1. An expander for a cryogenic refrigerator, said expander comprising:

an expander cylinder, a displacer type piston in said expander cylinder for dividing said expander cylinder into an expansion volume and compression volume, the area of said piston defining said expansion volume being greater than the area of said piston defining said compression volume, said expansion volume being the cold expansion volume which produces refrigeration upon gas expansion, a regenerator connected between such expansion and compression volumes, means for connecting a source of pulsating refrigerant gas to all said volumes, and means for urging said piston in a direction to first reduce said expansion volume, and increase said compression volume, and second to increase said expansion volume and reduce said compression volume, the improvement comprising:

an adjustable mechanical stop positioned to limit the stroke of said expander piston. said adjustable stop having means for controlling mechanical stop relative position between said expander cylinder and said expander piston so that said stop can be adjusted for a larger stroke of said expander piston during cooldown and to limit motion of said expander piston to a shorter stroke when said expander is operating near design temperature.

2. The expander of claim 1 wherein said adjustable stop is positioned to be directly in line with said expander piston during reciprocation of said expander piston.

3. An expander for a cryogenic refrigerator, said expander comprising:

an expander cylinder, a displacer type piston in said expander cylinder for dividing said expander cylinder into an expansion volume and compression volume, the area of said piston defining said expansion volume being greater than the area of said piston defining said compression volume. said expansion volume being the cold expansion volume which produces refrigeration upon gas expansion. a regenerator connected between said expansion and compression volumes, means for connecting a source of pulsating refrigerant gas to all said volumes, and means for urging said piston in a direction to first reduce said expansion volume, and increase said compression volume, and second to increase said expansion volume and reduce said compression volume, the improvement comprising:

an adjustable stop having a stop nose having an ex tended position where it lies in the path of said expander piston and limits the stroke of said expander piston and a retracted position at which said ex pander piston has a greater stroke so that said stop can be adjusted for a larger stroke of said expander piston during cooldown and for a shorter stroke when said expander is operating near design temperature.

4. An expander comprising:

an expander cylinder, an expander displacer type piston reciprocally mounted within said expander cylinder for separating said expander cylinder into a cold first expander volume and a second compression volume, means for supplying refrigerant gas to said second expander volume, a regenerator connected between said first and second expander volumes, the improvement comprising:

an adjustable mechanical stop positioned in the path of motion of said expander piston, said adjustable stop being a mechanical stop stronger than the driving forces on said expander piston to adjustably positively control the stroke of said expander piston so that said expander piston can have a greater expander stroke during cooldown and a lesser expander stroke when operating near cryogenic temperatures.

5. The expander of claim 4 wherein said adjustable stop is positioned to be directly in line with said expander piston during reciprocation of said expander piston 6. An expander comprising:

an expander cylinder, an expander displacer type piston reciprocally mounted within said expander cylinder for separating said expander cylinder into a cold first expander volume and a second compres sion volume, means for supplying refrigerant gas to said second expander volume, a regenerator connected between said first and second expander volumes, the improvement comprising:

an adjustable stop positioned in the path of motion of said expander piston said adjustable stop having a stop nose having an extended position where it lies in the path of said expander piston and limits the stroke of said expander piston and a retracted position at which said expander piston has a greater stroke so that said expander piston has a greater expanded stroke during cooldown and a lesser expanthe beginning of cooldown; and

inserting a mechanical stop to limit the length of stroke of the expander piston at design temperature so that cooldown time is reduced by a greater expansion ratio at beginning of cooldown and losses are reduced at cryogenic temperature by re duced expander piston stroke. 

1. An expander for a cryogenic refrigerator, said expander comprising: an expander cylinder, a displacer type piston in said expander cylinder for dividing said expander cylinder into an expansion volume and compression volume, the area of said piston defining said expansion volume being greater than the area of said piston defining said compression volume, said expansion volume being the cold expansion volume which produces refrigeration upon gas expansion, a regenerator connected between such expansion and compression volumes, means for connecting a source of pulsating refrigerant gas to all said volumes, and means for urging said piston in a direction to first reduce said expansion volume, and increase said compression volume, and second to increase said expansion volume and reduce said compression volume, the improvement comprising: an adjustable mechanical stop positioned to limit the stroke of said expander piston, said adjustable stop having means for controlling mechanical stop relative position between said expander cylinder and said expander piston so that said stop can be adjusted for a larger stroke of said expander piston during cooldown and to limit motion of said expander piston to a shorter stroke when said expander is operating near design temperature.
 2. The expander of claim 1 wherein said adjustable stop is positioned to be directly in line with said expander piston during reciprocation of said expander piston.
 3. An expander for a cryogenic refrigerator, said expander comprising: an expander cylinder, a displacer type piston in said expander cylinder for dividing said expander cylinder into an expansion volume and compression volume, the area of said piston defining said expansion volume being greater than the area of said piston defining said compression volume, said expansion volume being the cold expansion volume which produces refrigeration upon gas expansion, a regenerator connected between said expansion and compression volumes, means for connecting a source of pulsating refrigerant gas to all said volumes, and means for urging said piston in a direction to first reduce said expansion volume, and increase said compression volume, and second to increase said expansion volume and reduce said compression volume, the improvement comprising: an adjustable stop having a stop nose having an extended position where it lies in the path of said expander piston and limits the stroke of said expander piston and a retracted position at which said expander piston has a greater stroke so that said stop can be adjusted for a larger stroke of said expander piston during cooldown and for a shorter stroke when said expander is operating near design temperature.
 4. An expander comprising: an expander cylinder, an expander displacer type piston reciprocally mounted within said expander cylinder for separating said expander cylinder into a cold first expander volume and a second compression volume, means for supplying refrigerant gas to said second expander volume, a regenerator connected between said first and second expander volumes, the improvement comprising: an adjustable mechanical stop positioned in the path of motion of said expander piston, said adjustable stop being a mechanical stop stronger than the driving forces on said expander piston to adjustably posiTively control the stroke of said expander piston so that said expander piston can have a greater expander stroke during cooldown and a lesser expander stroke when operating near cryogenic temperatures.
 5. The expander of claim 4 wherein said adjustable stop is positioned to be directly in line with said expander piston during reciprocation of said expander piston.
 6. An expander comprising: an expander cylinder, an expander displacer type piston reciprocally mounted within said expander cylinder for separating said expander cylinder into a cold first expander volume and a second compression volume, means for supplying refrigerant gas to said second expander volume, a regenerator connected between said first and second expander volumes, the improvement comprising: an adjustable stop positioned in the path of motion of said expander piston said adjustable stop having a stop nose having an extended position where it lies in the path of said expander piston and limits the stroke of said expander piston and a retracted position at which said expander piston has a greater stroke so that said expander piston has a greater expanded stroke during cooldown and a lesser expander stroke when operating near cryogenic temperatures.
 7. The method of reducing cooldown time of a cryogenic expander wherein the expander has an expander cylinder and an expander displacer type piston reciprocally mounted within the expander comprising the steps of: permitting maximum stroke of the expander piston at the beginning of cooldown; and inserting a mechanical stop to limit the length of stroke of the expander piston at design temperature so that cooldown time is reduced by a greater expansion ratio at beginning of cooldown and losses are reduced at cryogenic temperature by reduced expander piston stroke. 